Method for manufacturing electrochemical device, and electrochemical device

ABSTRACT

A method for manufacturing an electrochemical device includes the following steps: a step of preparing a positive electrode, the positive electrode including a first current collector and a positive electrode layer containing a conductive polymer; a step of preparing a negative electrode, the negative electrode including a second current collector and a negative electrode layer; and a step of sealing the positive electrode, the negative electrode, and an electrolytic solution in an exterior body. The step of preparing the positive electrode includes a step of holding the positive electrode in depressurized atmosphere and then introducing gas containing CO 2  as a primary component into the depressurized atmosphere.

RELATED APPLICATIONS

This application is a Divisional application of U.S. patent applicationSer. No. 15/924,289, filed on Mar. 19, 2018, which is a continuation ofthe PCT International Application No. PCT/JP2016/004275 filed on Sep.20, 2016, which claims the benefit of foreign priority of Japanesepatent application No. 2015-189602 filed on Sep. 28, 2015, the contentsall of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an electrochemical device using alithium ion, and a method for manufacturing the electrochemical device.

BACKGROUND

In a known electrochemical device using a lithium ion, a conductivepolymer is used as a material of a positive electrode, and a carbonmaterial in which a lithium ion is occluded is used as a material of anegative electrode. During the time of charging such an electrochemicaldevice, a lithium ion in an electrolytic solution is occluded in thenegative electrode, and an anion in the electrolytic solution is to bedoped into the positive electrode. During the time of discharging, alithium ion is released from the negative electrode into theelectrolytic solution, and an anion is undoped from the positiveelectrode into the electrolytic solution. Accordingly, in a charging anddischarging cycle, the negative electrode uses a lithium ion, and thepositive electrode uses an anion (see, Unexamined Japanese PatentPublication No. 2014-123641, International Publication No. WO2007/88604, Unexamined Japanese Patent Publication No. H01-112658, forexample).

SUMMARY

A method for manufacturing an electrochemical device according to afirst aspect of the present disclosure includes the steps of; producinga positive electrode in which a positive electrode layer containing aconductive polymer is formed on a first current collector; producing anegative electrode in which a negative electrode layer containing amaterial configured to occlude and release a lithium ion is formed on asecond current collector; producing a laminated body in which aseparator is interposed between the positive electrode and the negativeelectrode; and sealing the laminated body together with an electrolyticsolution in an exterior body. The step of producing the positiveelectrode includes a step of holding the positive electrode indepressurized atmosphere and then introducing gas containing CO₂ as aprimary component into the depressurized atmosphere.

A method for manufacturing an electrochemical device according to asecond aspect of the present disclosure includes the steps of; producinga positive electrode in which a positive electrode layer containing aconductive polymer is formed on a first current collector; producing anegative electrode in which a negative electrode layer containing amaterial configured to occlude and release a lithium ion is formed on asecond current collector; producing a laminated body in which aseparator is interposed between the positive electrode and the negativeelectrode; and sealing the laminated body together with an electrolyticsolution in an exterior body. The step of producing the laminated bodyincludes a step of holding the laminated body in depressurizedatmosphere and then introducing gas containing CO₂ as a primarycomponent into the depressurized atmosphere.

An electrochemical device according to a third aspect of the presentdisclosure includes: a positive electrode in which a positive electrodelayer containing a conductive polymer is formed on a first currentcollector; a negative electrode in which a negative electrode layercontaining a material having an occluded lithium ion is formed on asecond current collector; a separator interposed between the positiveelectrode and the negative electrode; and an electrolytic solutioncontaining a lithium ion and an anion. An absorption spectrum for thepositive electrode layer which is measured by infrared spectroscopy hasa profile that a ratio (A₈₀₀/A_(max)) of an absorbance (A₈₀₀) at 800cm⁻¹ to a proximity peak absorbance (A_(max)) near 800 cm⁻¹ is greaterthan 0.30, a ratio (B₁₀₈₅/B_(max)) of an absorbance (B₁₀₈₅) at 1085 cm⁻¹to a proximity peak absorbance (B_(max)) near 1085 cm⁻¹ is greater than0.21, and a ratio (C₁₃₄₀/C_(max)) of an absorbance (C₁₃₄₀) at 1340 cm⁻¹to a proximity peak absorbance (C_(max)) near 1340 cm⁻¹ is greater than0.70.

A proximity peak means an absorption peak having maximum absorbanceamong absorption peaks appearing near each wavelength (800 cm⁻¹, 1085cm⁻¹, 1340 cm⁻¹) in an absorption spectrum measured by infraredspectroscopy. The occlusion and release of a lithium ion mean reversibleentering of a lithium ion into between multiple layers of a crystalstructure of the carbon material.

According to the present disclosure, an electrochemical device capableof suppressing a capacitance decrease, an internal resistance increase,and gas generation, and having improved reliability can be easilyobtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partially cutout perspective view of an electrochemicaldevice according to an exemplary embodiment of the present disclosure.FIG. 2 shows a FT-IR spectrum measured for a positive electrode layer ofeach cell.

FIG. 3A is a graph showing a capacitance change rate with regard to eachof cells A2, B2, and X2 in a float test.

FIG. 3B is a graph showing a direct-current resistance change rate withregard to each of cells A2, B2, and X2 in a float test.

FIG. 3C is a graph showing a gas change amount with regard to each ofcells A2, B2, and X2 in a float test.

FIG. 4A is a graph showing a capacitance change rate with regard to eachof cells B3 and X3 in a float test. FIG. 4B is a graph showing adirect-current resistance change rate with regard to each of cells B3and X3 in a float test.

FIG. 4C is a graph showing an L-scale change amount with regard to eachof cells B3 and X3 in a float test.

DESCRIPTION OF EMBODIMENT

Prior to describing exemplary embodiments of the present disclosure,problems in a conventional electrochemical device are described.

In such an electrochemical device using a lithium ion, water inside theelectrochemical device affects, for example, a capacitance and aninternal resistance at the time of charging and discharging. Thus, ithas been considered that, when an electrode using a conductive polymeras the positive electrode material is manufactured, the amount ofcontained water in the electrode is reduced by, for example, vacuum dryor immersion dehydration. The vacuum dry is holding the electrode indepressurized atmosphere for a predetermined time. The immersiondehydration is immersing the electrode in a non-aqueous solutioncontaining almost no water.

However, an electrochemical device using a positive electrode containinga conductive polymer provided with the process of reducing water asdescribed above does not exhibit a sufficient battery characteristic,and thus, is required to achieve improved reliability.

The present disclosure is intended to solve the above-described problem,and provides an electrochemical device having an excellent batterycharacteristic and improved reliability, and a method for manufacturingthe electrochemical device.

An exemplary embodiment of the present disclosure will be describedbelow.

FIG. 1 is a partially cutout perspective view of an electrochemicaldevice according to an exemplary embodiment of the present disclosure.In the electrochemical device according to the present exemplaryembodiment, electrochemical element 1 is housed together with anelectrolytic solution (not illustrated) inside exterior body 2 having abottomed tubular shape and made of, for example, aluminum (Al). Sealingmember 3 made of, for example, rubber is inserted in an opening end ofexterior body 2. And drawing fabrication and curling fabrication areprovided with the opening end of exterior body so as to achieve sealingof inside. Electrochemical element 1 is an example of laminated bodyaccording to the present disclosure.

The electrolytic solution is a mixture of an electrolyte such as LiPF₆,and a solvent obtained by mixing propylene carbonate (PC) and dimethylcarbonate (DMC) at a weight ratio of 1:1. In this case, a lithium ion(Li⁺) and PF₆ ⁻ exist in the electrolytic solution, and PF₆ ⁻ is anexample of anion according to the present disclosure.

Electrochemical element 1 has a configuration in which positiveelectrode 4 and negative electrode 5 are wound with separator 6 made of,for example, cellulose interposed between positive electrode 4 andnegative electrode 5. In positive electrode 4, positive electrode layer4 a made of, for example, polyaniline is formed on front and backsurfaces of current collector 4 a made of, for example, A1 foil. Innegative electrode 5, negative electrode layer 5 b containingnon-graphitizable carbon (hard carbon) is formed on front and backsurfaces of current collector 5 a made of, for example, copper (Cu)foil. Positive electrode 4 and negative electrode 5 are connected withlead wires 7, 8, respectively. Lead wires 7, 8 are externally extendedthrough through-holes 3 a, 3 b provided in sealing member 3. Currentcollectors 4 a, 5 a are examples of first and second current collectors,respectively, according to the present disclosure. The polyaniline andhard carbon are an example of conductive polymer and an example ofcarbon material, respectively, according to the present disclosure.

Next, a process of manufacturing the electrochemical device according tothe present exemplary embodiment will be described.

Positive electrode 4 (positive electrode layer 4 b) is formed bydepositing polyaniline directly on current collector 4 a throughelectrolytic polymerization. Accordingly, positive electrode layer 4 acontains no conduction agent or no binding agent.

After depositing the polyaniline on current collector 4 a, a drying step(first drying step) of removing water adsorbed in the polyaniline isperformed. Specifically, positive electrode 4 is held in depressurizedatmosphere for a predetermined time (vacuum-dried). Thereafter, atreturning to normal pressure, gas containing carbon dioxide (CO₂) as aprimary component is introduced (purged) into the depressurizedatmosphere.

Negative electrode 5 is formed by applying a carbon paste onto currentcollector 5 a and drying the carbon paste. The carbon paste was producedby kneading, with water, the hard carbon and, for example, a conductionagent and a binding agent. Accordingly, negative electrode layer 5 bcontains, for example, the conduction agent and the binding agent.

In addition, a lithium thin film is formed on negative electrode layer 5b by, for example, vacuum evaporation coating. The formation allowslithium pre-doping to be described later.

After positive electrode layer 4 a and negative electrode layer 5 b areformed, each of positive electrode layer 4 a and negative electrodelayer 5 b is partially removed so as to expose respective part ofcurrent collectors 4 a, 5 a. And then lead wires 7, 8 are joined toexposed surfaces of current collectors 4 a, 5 a by, for example,resistance welding or ultrasonic welding.

Water adsorbed in separator 6 is removed by holding separator 6 indepressurized atmosphere at a temperature heated up to approximately100° C. for a predetermined time. Thereafter, an atmosphere surroundingseparator 6 is returned to normal pressure by purging dry air into thedepressurized atmosphere at room temperature.

Thereafter, positive electrode 4, separator 6, negative electrode 5, andseparator 6 are laminated in this order so that positive electrode layer4 a and negative electrode layer 5 b face to each other with separator 6interposed between positive electrode layer 4 a and negative electrodelayer 5 b. The laminated body is then wound from an end part so thatpositive electrode 4 is positioned on an inner side, thereby formingelectrochemical element 1.

Subsequently, electrochemical element 1 is inserted into exterior body2, and then another drying step (second drying step) is performed.Similarly to the above-described first drying step, the second dryingstep is performed by holding electrochemical element 1 and exterior body2 in depressurized atmosphere for a predetermined time. Thereafter, anatmosphere surrounding electrochemical element 1 and exterior body 2 isreturned to normal pressure by purging gas containing CO₂ as a primarycomponent.

After that, exterior body 2 is filled with the electrolytic solution,and the entire electrochemical element 1 is impregnated with theelectrolytic solution by holding exterior body 2 filled with theelectrolytic solution in depressurized atmosphere.

Lastly, an atmosphere surrounding exterior body 2 is returned to normalpressure by purging dry air into the depressurized atmosphere.Thereafter, while lead wires 7, 8 are inserted in the respectivethrough-holes 3 a, 3 b, sealing member 3 is inserted into exterior body2 from the opening end side. And a vicinity of an opening end ofexterior body 2 is provided with drawing fabrication and curlingfabrication so as to seal the inside.

Accordingly, the electrochemical device according to the presentexemplary embodiment is produced.

As described above, in the present exemplary embodiment, gas containingCO₂ as a primary component is purged when the depressurized atmospherein which positive electrode 4 and electrochemical element 1 are held isreturned to normal pressure in the first drying step and the seconddrying step. Accordingly, in each of the first and second drying steps,water contained in the conductive polymer (polyaniline) in positiveelectrode layer 4 b is removed, and CO₂ is adsorbed into the surface ofpositive electrode layer 4 b. As described later, the inventors of thepresent disclosure have found that the occurrence of capacitancedegradation and internal resistance increase in a float test can besuppressed by performing the first and second drying steps.

Although a mechanism of such an effect according to the presentdisclosure is not clear, it is thought that the effect is achieved, notdirectly by adsorption of CO₂, but by generation of H₂CO₃ throughreaction of CO₂ with a slight amount of water (H₂) remaining in theelectrolytic solution. H₂CO₃ further changes into HCO₃ ⁻ and CO₃ ²⁻ insome cases. It is thought that these H₂CO₃, HCO₃, and CO₃ ²⁻ areadsorbed or doped on the surface of polyaniline to suppress furtheradsorption of H₂O, thereby reducing the influence of H₂O. Thisphenomenon can be confirmed as change in an absorption spectrum beforeand after CO₂ purging by Fourier transform infrared spectroscopy (FT-IR)as described later.

The gas introduced into the depressurized atmosphere is preferably gasonly containing CO₂, but may be gas containing CO₂ as a primarycomponent, in other words, gas containing CO₂ of 50% or higher with nomoisture. Next, pre-doping of negative electrode 5 will be described.The pre-doping described herein is occlusion of a lithium ion into thecarbon material (hard carbon) contained in negative electrode 5(negative electrode layer 5 b) before charging and discharging of theelectrochemical device. As described above, after the lithium thin filmis formed on negative electrode layer 5 b, electrochemical element 1 ishoused together with the electrolytic solution in exterior body 2 andleft to stand for a predetermined time. Thereby, lithium in the lithiumthin film becomes ionized and enters between layers of a crystalstructure of the carbon material (hard carbon) contained in negativeelectrode layer 5 b so as to form interlayer compounds of carbon atomsand lithium atoms. Accordingly, a lithium ion is occluded in the carbonmaterial through such a phenomenon, and the pre-doping is completed.

When a lithium ion is occluded in the carbon material contained innegative electrode 5 (negative electrode layer 5 b), an electrodepotential of negative electrode 5 decreases due to electrochemicalreaction of the lithium ion. Accordingly, a potential difference betweenpositive electrode 4 and negative electrode 5 increases, which leads toimproved energy density of the electrochemical device.

The pre-doping of a negative electrode is also performed in the field oftypical lithium ion secondary batteries. This pre-doping is intended toreduce irreversible capacity of the negative electrode in a charging anddischarging cycle to achieve improved charging and discharging capacity.However, the pre-doping of the electrochemical device according to thepresent disclosure is intended to increase the potential differencebetween positive electrode 4 and negative electrode 5 due to thepotential reduction of negative electrode 5. By the difference in theabove intentions, occlusion amounts of lithium ion in the above cases ofpre-doping are different with each other. Specifically, the occlusionamount of lithium ion in a typical lithium ion secondary battery onlyneeds to be sufficient for the irreversible capacity of the negativeelectrode, and thus is clearly smaller than the occlusion amount oflithium ion in the electrochemical device according to the presentdisclosure. In the present exemplary embodiment, a thickness of thelithium thin film is adjusted so that the occlusion amount is 80%approximately of a maximum occlusion amount capable in the negativeelectrode. The occlusion amount preferably ranges approximately from 50%to 95%, inclusive.

In the above-described exemplary embodiment, CO₂ is purged in any of thefirst drying step and the second drying step, but the present disclosureis not limited to this configuration. CO₂ may be purged only in one ofthe first and second drying steps. In this case, in the other of thefirst and second drying steps in which CO₂ is not purged, dry air andinert gas are preferably purged for returning to normal pressure. WhenCO₂ is purged, it is preferable to perform heating in a range from 15°C. to 120° C., inclusive. It is thought that, with this configuration,adsorption or doping of CO₂ into the conductive polymer is efficientlyperformed without heat damage on the positive electrode or any othercomponent.

In the above-described exemplary embodiment, the conductive polymer ispolyaniline, but the present disclosure is not limited to thisconfiguration. The conductive polymer may be a derivative ofpolyaniline. Alternatively, any other conductive polymers such as,polypyrrole, polythiophene, polyphenylene, or their derivatives may beused, or a plurality of them that are contained together may be used.

In the above-described exemplary embodiment, hard carbon as a carbonmaterial is used as a negative electrode active material of negativeelectrode layer 5 b, but it is preferable to use a material thatelectrochemically occludes and releases a lithium ion. For example, thenegative electrode active material of negative electrode layer 5 b maybe a carbon material such as non-graphitizable carbon (hard carbon),graphite, or easily graphitizable carbon (soft carbon), a hydrocarbonmaterial such as polyacene, a metal compound such as silicon oxide ortin oxide, an alloy such as a silicon alloy or a tin alloy, or aceramics material such as lithium titanate or manganate lithium. Thesematerials may be used alone or in combination of two kinds or more.Among these materials, a carbon material is preferable because itachieves low potential of the negative electrode.

In the above-described exemplary embodiment, the conductive polymer isdirectly deposited on current collector 4 a through electrolyticpolymerization, but the present disclosure is not limited to thisconfiguration. A conductive polymer formed in advance throughelectrolytic polymerization or chemical polymerization may be appliedand formed on current collector 4 a. In this case, a mixture of, forexample, conduction agent and binding agent can be applied together withthe conductive polymer, and the positive electrode layer is formed oftheir composite.

In the above-described exemplary embodiment, the mixture of electrolytesuch as LiPF₆ and a solvent obtained by mixing PC and DMC at a weightratio of 1:1 is used as the electrolytic solution, but the presentdisclosure is not limited to this configuration. Specifically, theelectrolyte may be a material that contains a lithium ion as a cationand any other anion, such as LiClO₄, LiBF₄, or LiAsF₆. The solvent usedfor the electrolytic solution may include cyclic carbonate, chaincarbonate, cyclic ester, chain ester, cyclic ether, chain ether, or anorganic solvent containing an epoxy group, a sulfone group, a vinylgroup, a carbonyl group, an amide group, or a cyano group. And it may beused by selecting (mixing) one kind, two kinds, or more from amongethylene carbonate, gamma butyrolactone, sulfolane, ethyl methylcarbonate, diethyl carbonate, and butylene carbonate as appropriate.

In the above-described exemplary embodiment, the lithium thin film isformed on negative electrode layer 5 b by vacuum evaporation coating,but the present disclosure is not limited to this configuration. Lithiumfoil formed by pressure rolling may be disposed on negative electrodelayer 5 b.

In the above-described exemplary embodiment, electrochemical element 1has a configuration in which a laminated body of positive electrode 4,negative electrode 5, and separator 6 is wound, but may have aconfiguration in which positive electrode 4, negative electrode 5, andseparator 6 are only laminated. The exterior body 2 may have aconfiguration in which circumferences of two sheets are closelycontacted with each other to seal inside, what is called a laminatestructure.

In the above-described exemplary embodiment, since the conductivepolymer is directly deposited on current collector 4 a by electrolyticpolymerization, positive electrode layer 4 a contains no binder norbinding material but only contains the conductive polymer. Contrast tothis, under a conventional manufacturing condition, a large amount ofwater is likely to be adsorbed into positive electrode layer 4 b. In theabove-described exemplary embodiment, cellulose, which is likely toadsorb water, is used as the material of separator 6. In the case of anelectrochemical device having a configuration with which water is likelyto be adsorbed, characteristic degradation is more likely to occur.Thus, the present disclosure is effective particularly for such aconfiguration to which water adsorption is likely to occur.

The shape, material, formation method, and fabrication method for anyother component may be changed as appropriate without departing from thescope of the present disclosure.

EXAMPLES

The present disclosure will be described further in detail based onexamples.

(Experiment 1)

Three cells (cell A1, cell B1, and cell X1) were produced to evaluateeffects of the CO₂ purge in the first drying step and the second dryingstep described above. The configurations and production methods of thethree cells are identical to each other unless otherwise stated in thefollowing description.

[Production of Positive Electrode]

An A1 foil having a plane shape of 2 cm×2 cm was used as a positiveelectrode collector (the first current collector). Polyaniline wassynthesized on front and back surfaces of the A1 foil by a galvanostaticelectrolytic polymerization method using an aqueous solution containingan aniline monomer of 1 mol/l and sulfuric acid of 2 mol/l aselectrolytic polymerization liquid. After the electrolyticpolymerization, the A1 foil was cleaned with distilled water and driedso as to form positive electrode layers on the front and back surfacesof the A1 foil. Each of positive electrode layers had a thickness of 60μm.

Subsequently, a part of each of positive electrode layers was removed toexpose the A1 foil, and an A1 tab lead was attached onto the exposedregion of the A1 foil by ultrasonic welding. Thereafter, a drying stepof heating the A1 foil to 100° C. in a vacuum container and holding theA1 foil in the container for 12 hours was performed. Then, theatmosphere in the container was returned to room temperature and normalpressure by purging with different gasses at 100° C. Specifically, thepurging gas for cell A1 was CO₂, and the purging gas for each of cell B1and cell X1 was dry air having a dew point of −40° C. or lower.

[Production of Negative Electrode]

A Cu foil having a plane shape of 2 cm×2 cm was used as a negativeelectrode collector (the second current collector). Carbon paste wasproduced by kneading a mixed powder and water at a weight ratio of40:60. The mixed powder is a mixture of hard carbon of 97 wt %, carboxylmethyl cellulose of 1 wt %, and styrene butadiene rubber of 2 wt. Andthen the carbon paste was applied on one surface of the Cu foil anddried to form a negative electrode layer having a thickness of 35 μm.

Thereafter, a drying step of heating the Cu foil to 110° C. in a vacuumcontainer and holding the Cu foil for 12 hours was performed. Then, theatmosphere in the container was returned to room temperature andreturned to normal pressure by purging with dry air having a dew pointof −40° C. or lower. In addition, a lithium thin film was formed on theentire surface of the negative electrode layer by vacuum evaporationcoating. Subsequently, a part of the negative electrode layer and thelithium thin film was removed to expose the Cu foil, and a Ni tab leadwas attached onto the exposed region of the Cu foil by resistancewelding.

[Production of Separator]

The separator was produced as follows. A cellulose sheet having athickness of 35 μm was cut out into a predetermined size and providedwith a drying step of heating the cellulose sheet to 110° C. in a vacuumcontainer and holding the cellulose sheet for 12 hours. Thereafter, theatmosphere in the container was returned to room temperature andreturned to normal pressure by purging with dry air having a dew pointof −40° C. or lower.

[Production of Electrolytic Solution]

Vinylene carbonate (VC) of 0.2 wt % was added to a solution obtained bymixing propylene carbonate (PC) and dimethyl carbonate (DMC) at a weightratio of 1:1. This mixture was used as a solvent for a solutioncontaining LiPF₆ of 1 mol/l as electrolyte, thereby obtaining anelectrolytic solution.

[Production of Electrochemical Element]

An electrochemical element was produced by laminating the positiveelectrode, the negative electrode, and the separator such that thepositive electrode layer and the negative electrode layer faces to eachother with the separator interposed between the positive electrode layerand the negative electrode layer. In other words, the positiveelectrode, the negative electrode, and the separator were laminated inan order of the negative electrode, the separator, the positiveelectrode, the separator, and the negative electrode.

[Production of Cell]

The electrochemical element was inserted into an exterior body composedof two A1 laminate sheets sealed to each other at three sides inadvance. Subsequently, a drying step of holding the exterior body for 30minutes at room temperature in a vacuum container was performed, andthen the atmosphere in the container was returned to normal pressure bypurging with different gasses. Specifically, the purging gas for each ofcell A1 and cell B1 was CO₂, and the purging gas for cell X1 was dry airhaving a dew point of −40° C. or lower.

Thereafter, the exterior body was filled with the electrolytic solution,and the entire electrochemical element was impregnated with theelectrolytic solution by holding the exterior body filled with theelectrolytic solution in depressurized atmosphere. Lastly, an atmospheresurrounding the exterior body was returned to normal pressure by purgingwith dry air having a dew point of −40° C. or lower. In addition, theremaining one side of exterior body was sealed so as to insulate the twotab leads from the exterior body. In this manner, cells A1, B1, and X1were produced.

Subsequently, the produced cells were evaluated as follows.

[IR Measurement]

The produced cells were each charged to 3.0 V at a current value (10C),which is ten times greater than current capacity, and then kept at theconstant voltage of 3.0 V for 30 minutes. Thereafter, theelectrochemical element was taken out of the exterior body underatmosphere having a dew point of −40° C. or lower, and disassembled intothe positive electrode, the negative electrode, and the separator. Then,the positive electrode was cleaned with DMC to remove any electrolyticsolution component. The positive electrode thus processed wasvacuum-dried to remove the DMC, and then analysis was performed on thepositive electrode layer of each cell by FT-IR. The FT-IR measurementwas performed by an ATR method.

FIG. 2 shows a FT-IR spectrum measured for the positive electrode layerof each cell. As for cells A1 and B1, an absorbance (A₈₀₀) at 800 cm⁻¹,an absorbance (B₁₀₈₅) at 1085 cm⁻¹, and an absorbance (C₁₃₄₀) at 1340cm⁻¹ are increased as compared to cell X1 as indicated with arrows inFIG. 2. Absorption at 800 cm⁻¹ indicates deformation vibration of CO₃,absorption at 1085 cm⁻¹ indicates deformation vibration of COH, andabsorption at 1340 cm⁻¹ indicates stretching vibration of C(OH)₂. Thus,as for cells A1 and B1, it is thought that at least one of H₂CO₃, HCO₃⁻, and CO₃ ²⁻ as derivatives of CO₂ is adsorbed or doped on the surfaceof polyaniline in the positive electrode layer by purging with CO₂.

A ratio (A₈₀₀/A_(max)) of A₈₀₀ to a proximity peak absorbance (A_(max))near 800 cm⁻¹, a ratio (B₁₀₈₅/B_(max)) of B₁₀₈₅ to a proximity peakabsorbance (B_(max)) near 1085 cm⁻¹, and a ratio (C₁₃₄₀/C_(max)) ofC₁₃₄₀ to a proximity peak absorbance (C_(max)) near 1340 cm⁻¹ werecalculated for each cell. Each position of main proximity peaks wasindicated with black triangles in FIG. 2. Specifically, a proximity peaknear 800 cm⁻¹ is a peak having a maximum absorbance among absorptionpeaks appearing between 700 cm⁻¹ to 900 cm⁻¹. A proximity peak near 1085cm⁻¹ is a peak having a maximum absorbance among absorption peaksappearing between 900 cm⁻¹ to 1200 cm⁻¹. A proximity peak near 1340 cm⁻¹is a peak having a maximum absorbance among absorption peaks appearingbetween 1200 cm⁻¹ to 1400 cm⁻¹. Table 1 lists the above-describedabsorbance ratios of each cell.

TABLE 1 Absorbance ratio Cell A1 Cell B1 Cell X1 A₈₀₀/A_(max) 0.55 0.570.30 B₁₀₈₅/B_(max) 0.41 0.49 0.21 C₁₃₄₀/C_(max) 0.86 0.83 0.70

As table 1 shows, the three absorbance ratios for cells A1 and B1 aregreater than the three absorbance ratios for cell X1. In other words, bypurging with CO₂, A₈₀₀/A_(max) is greater than 0.30, B₁₀₈₅/B_(max) islarger than 0.21, and C₁₃₄₀/C_(max) is greater than 0.70.

(Experiment 2)

Next, cells A2, B2, and X2 same as cells A1, B1, and X1, respectively,were produced in which the size of the positive electrode and thenegative electrode in Experiment 1 was changed from the plane shape of 2cm×2 cm to a plane shape of 4 cm×5 cm. While a voltage of 3.5 V wasapplied to the produced cells at 60° C., a float test was performed toevaluate a change rate of capacitance (electrostatic capacitance), achange rate of direct-current resistance, and a change amount(generation amount) of gas inside the exterior body with time elapse.FIGS. 3A to 3C show results of the float test.

FIG. 3A is a graph showing a capacitance change rate with regard to eachof cells A2, B2, and X2 in a float test. FIG. 3B is a graph showing adirect-current resistance change rate with regard to each of cells A2,B2, and X2 in a float test. FIG. 3C is a graph showing a gas changeamount with regard to each of cells A2, B2, and X2 in a float test. Atcell X2, the capacitance decreased (AC decreased) and the direct-currentresistance increased (ADCR increased) as time elapsed. Contrast to this,at cells A2 and B2, the capacitance and the direct-current resistancewere relatively stable with small change rates as time elapsed. As forthe gas generation amount, at cell X2, a large amount of gas wasgenerated immediately after start of the test. Contrast to this, at cellA2, almost no change occurred (no gas was generated), and at cell B2,the gas generation was suppressed as compared to cell X2 although gasincreased with time. This indicates that each characteristic isstabilized by purging with CO₂ when the positive electrode or theelectrochemical element is dried, thereby improving reliability.

(Experiment 3)

Subsequently, a wound-type cell described in the above-describedexemplary embodiment was produced, and a float test same as the floattest in Experiment 2 was performed. Similarly to the above-describedexemplary embodiment except that soft carbon was used as the carbonmaterial of the negative electrode layer, an electrochemical element wasproduced and sealed in an exterior body having a diameter of 1.25 cm anda length of 4 cm. Any other production conditions of the positiveelectrode and the negative electrode were same as production conditionsin Experiment 1. Cell B3 was produced by purging the electrochemicalelement with CO₂ after drying (purging the positive electrode with dryair after drying), and cell X3 was produced by purging the positiveelectrode and the electrochemical element with dry air after drying.Similarly to Experiment 2, a float test was performed on the two cells.FIGS. 4A to 4C show results of the float test.

FIG. 4A is a graph showing a capacitance change rate with regard to eachof cells B3 and X3 in a float test. FIG. 4B is a graph showing adirect-current resistance change rate with regard to each of cells B3and X3 in a float test. FIG. 4C is a graph showing an L-scale changeamount with regard to each of cells B3 and X3 in a float test. Here, theL-scale change amount is a change amount of the length in thelongitudinal direction of the exterior body, and is a value that variesalong with the change amount of the gas generated in the exterior body.Similarly to FIGS. 3A to 3C, at cell X3, the capacitance decreased (ACdecreased) and the direct-current resistance increased (ADCR increased)as time elapsed Contrast to this, at cell B3, the capacitance and thedirect-current resistance were relatively stable with small changeratios as time elapsed. As for the gas generation amount, at cell X3, alarge amount of gas was generated (the L-scale change amount is greatlyincreased) immediately after start of the test. Contrast to this, atcell B3, almost no change occurred. Thus the gas generation at cell B3was suppressed as compared to cell X3. It was confirmed from this resultthat each characteristic is stabilized by purging with CO₂ when theelectrochemical element is dried, thereby improving reliability.

The electrochemical device according to the present disclosure has anexcellent characteristic at rapid charging and discharging, and isuseful as, for example, a hybrid-vehicle power source used forregeneration and backup.

What is claimed is:
 1. A method for manufacturing an electrochemicaldevice, the method comprising the steps of: preparing a positiveelectrode, the positive electrode including a first current collectorand a positive electrode layer containing a conductive polymer;preparing a negative electrode, the negative electrode including asecond current collector and a negative electrode layer; and sealing thepositive electrode, the negative electrode, and an electrolytic solutionin an exterior body, wherein the step of preparing the positiveelectrode includes a step of holding the positive electrode indepressurized atmosphere and then introducing gas containing CO₂ as aprimary component into the depressurized atmosphere.
 2. A method formanufacturing an electrochemical device, the method comprising the stepsof: preparing a positive electrode, the positive electrode including afirst current collector and a positive electrode layer containing aconductive polymer; preparing a negative electrode, the negativeelectrode including a second current collector and a negative electrodelayer; preparing a laminated body including the positive electrode andthe negative electrode; and sealing the positive electrode, the negativeelectrode, and an electrolytic solution in an exterior body, wherein thestep of preparing the laminated body includes a step of holding thelaminated body in depressurized atmosphere and then introducing gascontaining CO₂ as a primary component into the depressurized atmosphere.3. The method for manufacturing an electrochemical device according toclaim 1, wherein the negative electrode layer contains a materialconfigured to occlude and release a lithium ion.
 4. The method formanufacturing an electrochemical device according to claim 2, whereinthe negative electrode layer contains a material configured to occludeand release a lithium ion.
 5. The method for manufacturing anelectrochemical device according to claim 1, wherein a temperature atthe depressurized atmosphere ranges from 15° C. to 120° C., inclusive,when the gas is introduced.
 6. The method for manufacturing anelectrochemical device according to claim 2, wherein a temperature atthe depressurized atmosphere ranges from 15° C. to 120° C., inclusive,when the gas is introduced.
 7. The method for manufacturing anelectrochemical device according to claim 1, wherein the conductivepolymer contains polyaniline or a derivative of polyaniline.
 8. Themethod for manufacturing an electrochemical device according to claim 2,wherein the conductive polymer contains polyaniline or a derivative ofpolyaniline.